It is sometimes necessary or desirable to insert a liner into an existing underground pipe. For example, this may be done in order to regain use of a previously abandoned underground pipe.
To insert the liner, it is necessary to excavate to provide access to the underground pipe. Following this, a section of the underground pipe is removed to provide an open pipe end through which the liner can be inserted. The liner can then be fed from the surface into the excavation and into the open pipe end. This process can be carried out relatively easily if the liner is very flexible because a flexible liner can be relatively easily deformed into the compound curve necessary for feeding it from the surface into the open end of the underground pipe. One such technique is illustrated, by way of example, in Overmyer et al U.S. Pat. Nos. 4,003,112 and 4,104,774.
Unfortunately, for certain applications, liners for underground pipes have thick walls and are very difficult to deform. Under these circumstances, the liner cannot be sharply bent as shown, by way of example, in the Overmyer et al patents cited above. To insert a stiff liner, it may be necessary to greatly elongate the excavation so that the stiff liner can be deformed into very gradual curves. However, it is desirable to minimize the excavation particularly when the underground pipe is located beneath a city street where a long excavation would do significant damage to the street and be costly to repair. In addition, regardless of the location of the underground pipe, it is generally desirable to minimize the time and costs for a project by minimizing the size of the excavation.